1. Field of the Invention
The present invention relates to the field of the crystal growth of epitaxial films and substrates. In particular the invention relates to the crystal growth of thermally mismatched epitaxy and substrate, and to the crystal growth of substrates that cannot be manufactured by traditional bulk growth techniques.
2. Description of Prior Art
As the scope of process manufacturing widens from mature areas such as semiconductor and ceramic processing to new material systems with new applications, areas of research and development become dominant bottlenecks where previously the technology proceeded quickly and naturally. Such is the case with the wide bandgap semiconductor, GaN, where because of the high melting point of the material and the high vapor pressure of N over the liquid or solid near this temperature, it is currently impossible to fabricate bulk GaN crystals of usable size for semiconductor manufacturing. Thus, GaN films are made by deposition of the GaN on a non-native substrate material, typically sapphire, where a large lattice mismatch and thermal mismatch exists. The lattice mismatch is the difference in length of the appropriate fundamental crystal length (the lattice constant) between the substrate and the epitaxy. The thermal mismatch is the difference in the crystal size of the substrate and epitaxy as function of temperature. The resulting material has large dislocation densities which limits the performance of electronic and optoelectronic devices fabricated from this films. Although engineers are slowly succeeding in working around these mismatch problems, the industry's progress would be increased with an available native substrate.
Even the epitaxial deposition of GaN has been difficult and not paralleled in similar compounds such as GaAs. To fully investigate the epitaxial process researchers has gone back to simple epitaxy techniques such as hydride-vapor-phase epitaxy (HVPE). In this technique large GaN growth rates and thicknesses can be produced. However, when the GaN epitaxy exceeds approximately 10 .mu.m the GaN films develop cracks upon cooling. These cracks are a mechanism to relieve the large strain that builds up in the GaN epitaxy-substrate system because of the thermal mismatch between the GaN film and that of the substrate material.
U.S. Pat. No. 5,679,152 to Tischler et al. discloses a method of making a single crystal GaN substrate by epitaxially depositing the GaN on a growth substrate. At the growth temperature the growth substrate is completely etched away, i.e. "sacrificed", either before or after the GaN deposition is complete. When the GaN layer is then cooled, there is no thermal mismatch because the sacrificial substrate is no longer present. This technique, however, has several disadvantages. Because the technique requires that the sacrificial substrate is completely removed at the high growth temperature, the deposition chamber must be divided into two and an etching process must be performed in one chamber while the deposition is performed in the other chamber. The additional etching required by this approach adds cost and complexity to the process. In addition, controlling these two processes and ensuring that they are properly isolated from each other can pose difficulties. It can also be difficult to hold the GaN layer in place after the sacrificial substrate is etched away.
There is additional prior art which falls into two categories: techniques that remove the unwanted substrate in a different process after growth, and techniques that allow the epitaxy to be grown strain-free, often called universal compliant substrates. An example of the former is the flip-chip bonding techniques, while examples of the latter are the epitaxial growth of materials on a thin Si film deposited on SiO.sub.2 deposited on Si (SOI), and the epitaxial growth on a processed thin film that are suspended as a membrane on a post structure above an underlayer (called a Universal Substrate). Flip-chip bonding allows the substrate of an epitaxially deposited film to be removed after cooling by mounting the substrate-epitaxy structure upside down and subsequently removing the substrate. This does not account for any thermal mismatch problem in the original substrate-epitaxial system. The thin Si film on SOI allows the epitaxially deposited material to nucleate on the extremely thin Si film which floats on the oxide at the deposition temperature. The effect is similar in the Universal substrate case: A lattice mismatched material can nucleate and grow on the extremely thin Si region of the substrate which is separated from the rest of the substrate by the liquid oxide. It is the substrate that quickly conforms to the epitaxy's materials properties. However, the oxide solidifies before completely cooling and thus the effective substrate is now quite thick and thermal mismatch is relieved by cracking in the epitaxy--not the substrate. The Universal substrate cannot effectively be used to solve thermal mismatch problems because the cracking of either of the suspended substrate or epitaxy is fatal.